The present invention relates to a low stress polyimide composition using as a coating material to a substrate having thermal expansion coefficient less than 30 ppm/.degree.C. and a precursor composition solution of the polyimide composition.
As a heat resistant material, polyimides are widely used for coating various substrates therewith. Various improvements have been proposed to enhance adhesion of polyimides to substrates without sacrificing heat resistance and other useful properties. However, a conventional polyimide film obtained by heat treatment has a high residual stress due to a large difference of thermal expansion coefficient between a substrate and the polyimide. With this, the film will tend to have deformation, peeling and breaking. To solve this problem, for example, JP-A-60-250031, JP-A-61-60725 and JP-A-62-184025 propose polyimides which have small positive values of thermal expansion coefficient due to hard main chain structures of the polyimide molecules. Polymer Preprints, Japan vol. 39, No. 3, 790 (1990) discloses a fluorine-containing polyimide prepared by reacting pyromellitic acid dianhydride with 2,2'-bis(trifluoromethyl)-4,4'-benzidine. This polyimide has a hard main chain structure and a negative thermal expansion coefficient. However, thermal expansion coefficient of this polyimide can be controlled within a range from -5.times.10.sup.-6 to 8.2.times.10.sup.-5 by copolymerizing other components or by blending other polymers.
The above-mentioned polyimides having hard main chain structures are unsatisfactory in adhesion to a substrate such as a glass, a ceramic or an organic material. Therefore, it is recommended to rough a substrate surface by, for example, plasma treatment. Furthermore, the above-mentioned polyimides having hard main chain structures are hard to be treated by wet etching process. JP-A-62-253621 discloses a polyimide which has a low thermal expansion coefficient and is treated by wet etching process. However, an etching solution for this polyimide is limited to only a solution containing hydrazine and ethylenediamine which is not preferable to be used from a view point of working environment. Furthermore, this polyimide is still unsatisfactory in adhesion.
JP-A-62-235382 discloses a thermosetting adhesive film prepared by compounding a polyimide and a polymaleimide. JP-A-3-247680 discloses a heat resistant adhesive film prepared by adding a bismaleimide compound to a polyamic acid solution. JP-A-4-198363 discloses a solvent resistant polyimide composition prepared by blending a polyimide having fluorine-containing substituent groups with a bismaleimide compound. JP-A-3-205474 discloses an adhesive composition prepared by adding a bismaleimide compound to a polyamic acid solution of which glass transition point after the completion of imidization reaction is not higher than 260 .degree. C. The above prior art relate to a polyimide or a polyamic acid, which has a flexible connecting group such as --O-- in its main chain and is large in thermal expansion coefficient. A polyimide which has a hard main chain and is small in thermal expansion is insoluble in any solvent, and has a high glass transition point which is usually not lower than 260.degree. C. Therefore, it is considered to be difficult to improve adhesive property by blending a bismaleimide compound through a conventional technique. Furthermore, it is not easy to treat a three-dimensionally cured bismaleimide by wet etching process. The above mixture containing a bismaleimide compound is mainly used as an independent type film, and it is not mentioned at all if the mixture can be treated by wet etching process or not.